1. Field of the Invention
The present invention relates to antennas used for wireless communications and to wireless communication devices.
2. Description of the Related Art
Recently, in the field of wireless communication devices, such as cellular phones, development for achieving multiple resonances or multiple bands is in progress in order to achieve wide bandwidths. Research studies are being carried out for antennas in which a plurality of resonant frequencies are controlled to allow transmission and reception with a wide bandwidth. Also, antennas in which a frequency can be changed to achieve a wide bandwidth are being considered.
Examples of such antennas that have been proposed include antennas disclosed in Patent Documents 1 to 3.
An antenna disclosed in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2003-51712), is an inverted-F-shaped antenna device. More specifically, an antenna element is disposed in parallel above a ground conductor, and at least one coupling element is provided in parallel between the ground conductor and the antenna element. The antenna element is electrically connected to the ground conductor via a short-circuiting conductor, and is connected to a feeding point of a feeding coaxial cable. By providing the coupling element in addition to the antenna element as described above, two resonant frequencies are obtained.
In an antenna disclosed in Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2002-232313), an antenna element and a variable capacitor are provided, the variable capacitor being connected in series or parallel with the antenna element to form a resonant circuit, and the control voltage is applied to the variable capacitor to change a resonant frequency.
In an antenna disclosed in Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2004-320611), a radiating element and a tuning circuit are connected in series. In the tuning circuit, a first inductor is connected in series with a parallel circuit including a variable capacitor. A first resonance frequency is obtained by a first antenna element and a second antenna element connected in series, and a second resonant frequency is obtained by the first antenna element alone. Furthermore, a third resonant frequency is obtained by a third antenna element provided from a feeding element.
However, the antennas according to the related art described above have the following problems.
Regarding the antenna disclosed in Patent Document 1, since the antenna is an inverted-F-shaped antenna device, when the antenna is mounted on a small and thin wireless communication device such as a cellular phone, the position of attachment of the coupling element is restricted to a low position because the height from the ground conductor to the antenna element must be small. Thus, restriction is imposed on the control of resonant frequencies of multiple resonances, so that the bandwidth can be increased only to approximately 1.5 times the bandwidth of an inverted-F antenna element. Also, the bandwidth ratio is approximately several percent at best.
Regarding the antenna disclosed in Patent Document 2, it is possible to control the resonant frequency according to the control voltage. However, since a frequency-changing resonance circuit implemented using a variable capacitor is provided in the proximity of a feeding section of the antenna element, the condition of matching between the feeding section and the antenna element changes. Thus, a complex matching circuit is needed. As contrasted with the above, an example where a frequency-changing resonance circuit is provided at a distal-end portion of an antenna element is disclosed. In this example, although a complex circuit configuration is not required, since the resonance circuit is provided at the distal-end portion of the antenna element, where the electric field is most intense (current density is smallest), it is not possible to change the resonant frequency greatly. Furthermore, a large control voltage is needed in order to change the resonant frequency of the antenna by a desired range by controlling a single variable capacitor. This does not allow for satisfaction of the demand for low-voltage operation required for a wireless communication device such as a cellular phone.
Regarding the antenna disclosed in Patent Document 3, it is possible to achieve multiple resonances and to change resonant frequencies. However, since the third antenna element is connected in parallel to the feeding element without an intervening tuning circuit, it is not possible to change the third resonant frequency significantly. Furthermore, since the parallel circuit is disposed in the proximity of a feeding section of the radiating element, the problems of the antenna disclosed in Patent Document 2 also exist.